The effectiveness and mechanistic study of using EDDS for metal extraction from contaminated soils

Heavy metal contamination in soil is widespread around the world. As a result, soil remediation is of high priority due to the toxicity and persistence of heavy metals in soil-water interface. [S,S]-ethylenediaminedisuccinic acid (EDDS) is an emerging chelating agent for enhancing heavy metal extraction. In this study, the performance of EDDS in soil remediation and/or the corresponding mechanisms have been examined through batch kinetics, mathematical modeling, and spectroscopic study. The results obtained in this study showed that when EDDS was deficient, initial unselective extraction of heavy metals (Cu, Zn and Pb) followed by Zn and Pb re-adsorption would occur. The interactions among metal–EDDS complexes were shown to be influenced by the characteristic of the metal center. Compared with ZnEDDS2- and PbEDDS2-, CuEDDS2- was the least adsorbed or exchanged, which may be attributed to higher ionic potential and electron configuration of Cu. Under excessive EDDS, the amount of extraction was independent of EDDS dosage and higher portion of extraction was observed when metals were less strongly bound with soils. Two mathematical models utilized the initial metal distribution in soils were developed and shown to provide a satisfactory description of the experimental results of the 7-d extraction kinetics and equilibrium of Cu, Zn and Pb for different types of soils. The types of interaction between chelating agents and soil components, iron oxides, were further investigated. The pH-dependent zwitterionic structure of a chelating agent (EDDS or EDTA for comparison) was suspected to play an important role on metal extraction and re-adsorption. Moreover, outer-sphere complexation of chelating agents was the dominant adsorption mechanism in the soil-water interface while in dehydrated condition, EDTA would be bound with iron oxides through inner-sphere complexation and no inner-sphere complexation was found for the dehydrated EDDS.